Add test against add/sub of zero with downwards rounding

Author: mborland

test/test_downward_rounding.cpp

@@ -78,5 +78,23 @@ int main()
     test_sub<decimal_fast128_t>("1e+2", "1e-383", "99.99999999999999999999999999999999");
     test_sub<decimal_fast128_t>("1e+1", "1e-383", "9.999999999999999999999999999999999");
 
+    test_add<decimal32_t>("9.999999e20", "0", "9.999999e20");
+    test_add<decimal_fast32_t>("9.999999e20", "0", "9.999999e20");
+
+    test_sub<decimal32_t>("9.999999e20", "0", "9.999999e20");
+    test_sub<decimal_fast32_t>("9.999999e20", "0", "9.999999e20");
+
+    test_add<decimal64_t>("9.999999999999999e200", "0", "9.999999999999999e200");
+    test_add<decimal_fast64_t>("9.999999999999999e200", "0", "9.999999999999999e200");
+
+    test_sub<decimal64_t>("9.999999999999999e200", "0", "9.999999999999999e200");
+    test_sub<decimal_fast64_t>("9.999999999999999e200", "0", "9.999999999999999e200");
+
+    test_add<decimal128_t>("9.999999999999999e200", "0", "9.999999999999999e200");
+    test_add<decimal_fast128_t>("9.999999999999999e200", "0", "9.999999999999999e200");
+
+    test_sub<decimal128_t>("9.999999999999999e200", "0", "9.999999999999999e200");
+    test_sub<decimal_fast128_t>("9.999999999999999e200", "0", "9.999999999999999e200");
+
     return boost::report_errors();
 }